1. Field of the Invention
The present invention relates to a data compressing apparatus and a data decoding apparatus in which a data compressing/decoding method such as LZSS or the like is expanded.
2. Related Background Art
A technique for transmitting data via a network in a short amount of time has been developed in association with the recent spread of the Internet. A data compression method intends to accomplish such an object by reducing the data itself to be transmitted.
In such a compression method, among lossless data compression methods, an origin of a dictionary coding which is most frequently used at present is a Lempel-Ziv encoding method that was created by Abraham Lempel and Jacob Ziv in 1977, and this encoding method is called a slide dictionary coding, LZ77, or the like. According to the LZ77, an input character data string which has been encoded before is used as a dictionary, and the longest coincidence with a continuous portion in the input character data string is searched, thereby encoding coincidence information. Since the LZ77 employs a null-pointer and since the first character which is outputted after the matching can be also used for the next matching, there is no need to output in such a case, and therefore, there is only a slight redundancy. After that, as a method of improving this encoding method, Storer and Szymanski created a data compression method called LZSS. Various techniques using such LZSS have been proposed (for example, refer to JP-A-5-241777, JP-A-7-273667, and JP-A-2000-315954).
The LZSS is a method whereby the pointer is outputted only when the character string is longer than a predetermined pointer length, and, in the other cases, the character data is outputted. Therefore, according to encoding in the LZSS, since the data in a form such that the encoded character data and pointer are mixed is outputted, and a specific bit (an extra ID-bit) is used to distinguish the character data and the pointer which are being outputted.
It has been known that when a compression ratio of the LZSS is compared with that of another compression method, the compression ratio to, particularly, character data having regularity is high. There is also a feature such that the decoding in the LZSS is performed at a high speed. For example, a large effect is obtained particularly in the case where the pointer is encoded by a method that is combined with the other entropy methods.
However, since the conventional data compression method is constructed by the character data and the pointer, it is difficult to embed data other than the character data, for example, information such as password or copyright information. Since the conventional data compression method is constructed only by the character data and the pointer, there is a limitation in the case of accomplishing a more excellent compression ratio in the range of those means.